252. By looking at the Moon with an ordinary telescope we perceive that
her surface is diversified with long tracts of prodigious high mountains
and deep cavities. Some of her mountains, by comparing their height with
her diameter (which is 2180 miles) are found to be three times higher
than the highest hills on our Earth. This ruggedness of the Moon’s
surface is of great use to us, by reflecting the Sun’s light to all
sides: for if the Moon were smooth and polished like a looking-glass, or
covered with water, she could never distribute the Sun’s light all
round; only in some positions she would shew us his image, no bigger
than a point, but with such a lustre as would be hurtful to our eyes.
[Sidenote: Why no hills appear on her edge.]
253. The Moon’s surface being so uneven, many have wondered why her edge
appears not jagged, as well as the curve bounding the light and dark
places. But if we consider, that what we call the edge of the Moon’s
Disc is not a single line set round with mountains, in which case it
would appear irregularly indented, but a large Zone having many
mountains lying behind one another from the observer’s eye, we shall
find that the mountains in some rows will be opposite to the vales in
others; and so fill up the inequalities as to make her appear quite
round: just as when one looks at an orange, although it’s roughness be
very discernible on the side next the eye, especially if the Sun or a
Candle shines obliquely on that side, yet the line terminating the
visible part still appears smooth and even.
[Illustration: Plate VII.
_J. Ferguson delin._ _J. Mynde Sculp._]
[Sidenote: The Moon has no twilight.